1. Technical Field
This invention relates to semiconductor packaging in general, and in particular, to making low-cost, thermally enhanced, chip-scale, lead-on-chip semiconductor packages.
2. Related Art
In a well-known type of semiconductor package, the back surface of a semiconductor die, or xe2x80x9cchip,xe2x80x9d is mounted on a metal die-attach pad contained in a substrate, e.g., a leadframe or a laminate, and surrounded by a plurality of metal leads contained therein. A plurality of fine, conductive wires are bonded between metal pads on an xe2x80x9cactive,xe2x80x9d front surface of the chip and the metal leads in the substrate to electrically interconnect the chip and substrate. The die, wire bonds, and portions of the substrate are then encapsulated in a protective plastic body.
The metal die-attach pad in the substrate gives the package relatively good thermal performance. However, the wire bonds between the chip and the substrate result in a relatively large package size. In an effort to reduce package size, so-called xe2x80x9cLead-On-Chipxe2x80x9d (xe2x80x9cLOCxe2x80x9d) packages were developed in which the leads of a leadframe substrate are attached to the active, upper surface of the-chip and wire bonded to the pads thereon with very short wire bonds, such as described by R. P. Pashby, et al., in U.S. Pat. No. 4,862,245.
Later variations on this LOC technique include, a direct attachment between the pads on the chip and the leads in a Tape Automated Bonding (xe2x80x9cTABxe2x80x9d) tape substrate, as described by K. Michii in U.S. Pat. No. 5,252,853; a xe2x80x9cflip-chipxe2x80x9d attachment between the pads on the chip and the leads in a metal leadframe substrate, as described by J. M. Wark in U.S. Pat. No. 5,817,540; and, a combination of short wire bonds and a flip-chip attachment between the chip pads and the leads of a leadframe substrate, as described by M. B. Ball in U.S. Pat. No. 5,917,242.
While the foregoing LOC packages achieve some reduction in package size due to the reduced size of the electrical connections between the die and the substrate, they do so at the expense of the thermal performance of the package, relative to the above packages in which the back side of the chip is attached to a metal die-attach pad in the substrate. Efforts to address this latter problem in LOC packages include etching thermal xe2x80x9cvias,xe2x80x9d i.e., openings, in the back side of the chips, or attaching a heat sink to the back side of the chip, as described by, e.g., C. P. Wyland in U.S. Pat. No. 5,986,885. However, these latter measures can largely offset the benefit of a reduced package size afforded by an LOC design, and in any case, add cost to the package.
This invention provides a low-cost, thermally enhanced, chip-scale, LOC semiconductor package, and a method for making it. The novel package includes a substrate with a plurality of metal lead fingers in it, and a semiconductor chip having an active surface with a plurality of ground, power, and signal connection pads thereon. The active surface of the chip is mounted on an upper surface of the substrate with the ground, power, and signal pads in a flip-chip electrical connection with corresponding ones of the lead fingers in the substrate.
The novel method includes locating at least two of the ground or the power connection pads on the chip in a central region thereof, where the operating temperature in the chip is the greatest. Corresponding metal lands are formed in the substrate at positions corresponding to the positions of the ground or the power pads on the chip, and are connected to the corresponding ground or power pads in the chip in a flip-chip electrical connection. The lower surfaces of the corresponding lands in the substrate are exposed to the environment through a lower surface of the package for attachment by, e.g., soldering, to an external heat sink, e.g., a heavy grounding or power pad on a mother board.
Advantageously, the corresponding lands in the substrate can be formed on or otherwise connected to selected ones of the lead fingers, viz., grounding or power fingers, and/or can be combined with each other into a single, large grounding or power land in the substrate to provide even greater thermal and electrical conductivity. The substrate can comprise a patterned metal layer laminated on a dielectric layer, e.g., a flexible tape or a fiberglass-epoxy resin composite, in which the lower surface of the lands are exposed through openings formed through a lower surface of the dielectric layer at positions corresponding to the positions of the lands.
Alternatively, the substrate can comprise a xe2x80x9cmicro-leadframexe2x80x9d (xe2x80x9cMLFxe2x80x9d) with a dielectric plastic body molded over it such that the lower surfaces of the grounding or power lands are exposed through, and optionally, flush with, a lower surface of the plastic body. The MLF can be made by forming a pattern of an etch-resistant material corresponding to the lead fingers and lands on a metal, then etching the desired pattern. In packages in which the desired spacing between the lead fingers and the lands is less than the thickness of the metal, the etching process may involve etching about half way through the metal, forming a second pattern of etch-resistant material on the half-etched portions of the metal, and then etching to produce the desired pattern.
The large, centrally located grounding or power lands in the substrate of the invention provide enhanced thermal and electrical connectivity between the chip and the external environment, thereby enabling a low-profile, flip-chip electrical connection method to be used in the package, and eliminating the need for a die-attach pad in the substrate, back-side thermal vias in the chip, or a heat sink on the back side of the chip, and accordingly, the invention is particularly well suited to the low-cost packaging of, among other types of devices, Thin Standard Outline Package (xe2x80x9cTSOPxe2x80x9d) Dynamic Random Access Memory (xe2x80x9cDRAMxe2x80x9d) devices.